Backside texturing by cusps to improve IR response of silicon solar cells and photodetectors

ABSTRACT

The absorption coefficient of silicon for infrared light is very low and most solar cells absorb very little of the infrared light energy in sunlight. Very thick cells of crystalline silicon can be used to increase the absorption of infrared light energy but the cost of thick crystalline cells is prohibitive. The present invention relates to the use of less expensive microcrystalline silicon solar cells and the use of backside texturing with diffusive scattering to give a very large increase in the absorption of infrared light. Backside texturing comprises a plurality of cusped features providing diffusive scattering. Constructing the solar cell with a smooth front surface results in multiple internal reflections, light trapping, and a large enhancement of the absorption of infrared solar energy.

This invention is a continuation-in-part, CIP, of U.S. nonprovisionalU.S. patent application Ser. No. 12/927,423 filed on 15 Nov. 2010, byLeonard Forbes, “Backside Nanoscale Texturing to Improve IR Response ofSilicon Solar Cells and Photodetectors”, which is based on U.S.Provisional Patent Application No. 61/285,416, by L. Forbes, filed on 10Dec. 2009, “Microcrystalline Silicon Solar Cell with Back Side Texturingfor Improved Infrared Absorption and Stacked Double Layer Solar Cells,”for which the priority date of 10 Dec. 2009 is claimed.

TECHNICAL FIELD

The present invention relates to silicon solar cells with improvedenergy conversion efficiency. More specifically, the present inventionrelates to backside texturing by cusps of silicon solar cells to providediffusive backside scattering and reflection of the incident red andinfrared light reaching the backside.

BACKGROUND

Texturing has been used as a technique for light trapping and to improvethe efficiency of photodetectors and solar cells due to multipleinternal reflections and light trapping. A portion of the knownliterature describes the use of backside texturing of photodiodes toimprove the absorption of near infrared light energy. One of the firstdescriptions of using this technique in photodetectors was by A. E. St.John in U.S. Pat. No. 3,487,223, “Multiple Internal Reflection Structurein a Silicon Detector which is Obtained by Sandblasting”. Anotherdescription was provided by J. E. Cotter, “Optical intensity of light inlayers of silicon with rear diffuse reflectors,” Journal of AppliedPhysics, vol. 84, no. 1, pp. 618-24, 1 Jul. 1998. A more recentdescription of the same technique on ultra-thin solar cells has beengiven by O. Berger, D. Inns and A. G. Aberle, “Commercial white paint asback surface reflector for thin-film solar cells,” Solar EnergyMaterials & Solar Cells, vol. 91, pp. 1215-1221, 2007, hereinafterreferred to as Aberle. Aberle disclosed the use of white paint as thebackside diffuse reflector. Aberle's initial results show a largeincrease in the absorption with the white paint as a back surfacereflector but only a modest ten or twenty percent increase in thequantum efficiency. Aberle's later results indicate that a twenty toforty percent enhancement in conversion efficiency can be obtained onthin film microcrystalline solar cells by adding more titanium dioxideto the white paint. However, thick layers of the order 80 μm or more ofpaint are required on thin film solar cells which in themselves are onlya few micrometers thick.

A recent analysis of the enhancement of infrared absorption in solarcells and photo detectors has been disclosed by L. Forbes and M. Y.Louie, “Backside Nanoscale Texturing to Improve IR Response of SiliconPhotodetectors and Solar Cells,” Nanotech, vol. 2, pp. 9-12, June 2010.Regularly textured surfaces have been described for solar cells, by A.Arndt, J. F. Allison, J. G. Haynos, and A. Meulenberg, Jr., “Opticalproperties of the COMSAT non-reflective cell,” 11th IEEE PhotovoltaicSpec. Conf., p. 40, 1975. The majority of solar cells use single frontside textured surfaces. Front side texturing serves to reduce thereflectivity of the silicon surface due to multiple attempts attransmission through the front surface as has been described by A.Arndt. U.S. Pat. No. 5,589,704 to Levine, “Article Comprising a Si-basedPhotodetector,” describes the front side texturing of a photodetector byplasma etching. Surface texturing of the illuminated side of solarcells, in the form of a regular, saw-tooth pattern has also beendescribed in U.S. Pat. No. 5,641,362 to Meier, “Structure andFabrication Process for an Aluminum Alloy Junction Self-aligned BackContact Silicon Solar Cell”. Further, U.S. Pat. No. 7,582,515, to Choiet al. “Multi-Junction Solar Cells and Methods and Apparatus for FormingSame” discloses a tandem configuration, which is a single piece ofsemiconducting material. A tandem arrangement as described herein meansa single substrate of semiconductor material in which processing hascreated various layers having individual characteristics of optical,electrical, and/or mechanical properties. U.S. Pat. No. 5,627,081 toTsuo, et al., describes porous silicon structures on the front side ofsubstrates to reduce reflectance of visible light. U.S. Pat. No.4,673,770 to Mandelkorn “Glass sealed silicon membrane solar cell”describes a ground and silvered bottom glass cover plate separated fromthe substrate but not structures etched into or deposited on to thesubstrate. U.S. Pat. No. 5,080,725 to Green, et al., “Optical propertiesof solar cells using tilted geometrical features,” describes ridges andpyramids etched into the substrate producing reflections only atspecific angles. Matsuyama et al., in U.S. Pat. No. 6,072,117, toMatsuyama et al. “Photovoltaic Device Provided with an Opaque SubstrateHaving a Specific Irregular Surface Structure”, disclose solar cells CVDdeposited upon opaque substrates with linear recesses. Methods ofcreating textured surfaces are described by S. W. Chang, V. P. Chuang,S. T. Boles, and C. V. Thompson, “Metal-Catalyzed Etching of VerticallyAligned Polysilicon and Amorphous Silicon Nanowire Arrays by EtchingDirection Confinement,” Advanced Functional Materials, vol. 20, no. 24,pp. 4364-4370, 2010, and by S. Brieger, O. Dubbers, S. Fricker, A.Manzke, C. Pfahler, A. Plettl and P. Ziemann, “An approach for thefabrication of hexagonally ordered arrays of cylindrical nanoholes incrystalline and amorphous silicon based on the self-organization ofpolymer micelles,” Nanotechnology, vol. 17, pp. 4991-4994, 2006,doi:10.1088/0957-4484/17/19/036.

One of the best examples of the scattering and diffraction of light isthe classical treatment of the interaction of light with narrow slitssuch as given by Born and Wolf, “Principles of Optics, 7^(th) Ed.,”Cambridge University Press, 1999, pp. 246-255.

From the foregoing it is apparent that there are conflictingrequirements for the absorption of infrared light energy and visiblelight energy. Front side texturing is desirable to maximize absorptionof visible light energy and the backside condition is irrelevant sincevisible radiation is strongly absorbed and utilized near the frontsurface. On the other hand, a smooth front side and textured backside isdesirable to maximize the utilization of infrared light energy.

Front side textured anti-reflecting layers, or antireflecting layers ontop of front side texturing are required to maximize absorption ofvisible light energy, the backside condition is irrelevant since visibleradiation is strongly absorbed near the front surface. On the other handa smooth front side and textured backside is needed to maximize theutilization of infrared light energy. Front side anti-reflecting layersthat transmit visible radiation can be used in conjunction with backsidediffusive texturing since these front side layers will be reflecting inthe infrared.

SUMMARY

Accordingly, a semiconductor solar cell or photodetector having improvedincident radiation absorption is disclosed, in which a silicon substratehas a substantially planar first surface available to the incidentradiation and a layer transparent to visible light and infraredwavelengths is disposed at the first surface. This layer is internallyreflective to infrared wavelengths of radiation scattered within thefirst semiconductor substrate. A textured layer is disposed at a secondsurface of the silicon substrate, the textured layer being a diffusiveradiation scattering layer for infrared wavelengths of radiation, and areflecting layer disposed on the textured layer and spaced apart fromthe second surface by the textured layer, whereby infrared wavelengthsof radiation are returned through the textured layer toward the firstsemiconductor substrate.

Another substrate substantially transparent to infrared incidentradiation may be stacked with the silicon substrate in a manner leavinga gap between the two substrates, such that a difference between theindex of refraction of a substrate and the gap results in infraredwavelength energy internally scattered is reflected back into the lattersilicon substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantage of the presentinvention, reference is being made to the following detailed descriptionof preferred embodiments and in connection with the accompanyingdrawings, in which:

FIG. 1 is an illustration of a solar cell or photodetector cross sectionwith scattered backside total reflection from a regularly texturedbackside and backside mirror-like reflector.

FIG. 2 is an illustration of a thin film microcrystalline solar cell.

FIG. 3 is a graph of external quantum efficiency versus incident energywavelength for a solar cell/photodetector with diffuse back surfacereflector or Pigmented Diffuse Reflector (PDR), white paint, and withouta diffusive backside reflector.

FIG. 4 is a graph of improvement in the infrared responsivity of asilicon solar cell/photodetector employing backside texturing and abackside mirror-like reflector.

FIG. 5 is a graph of responsivity of a 200 μm thick crystalline solarcell with planar front surface and with different amounts of diffusivescattering at the back surface.

FIG. 6 is a graph of responsivity of a thin microcrystalline solar cellwith different amounts of diffusive scattering at the back surface.

FIG. 7 is a cross sectional illustration of a thin film microcrystallinesilicon solar cell/photodetector which may employ the present invention.

FIG. 8 is a cross sectional illustration of stacked solarcells/photodetectors for absorption of visible light in the top cell andabsorption of infrared light in the bottom cell and which may employ thepresent invention.

FIG. 9A is an illustration of a solar cell or photodetector crosssection similar to that of FIG. 1, wherein cusped features disposed onthe silicon backside provides diffusive scattering.

FIG. 9B is an intensity versus distance graph of Lambertian scatteringfrom a single cusped feature, such as that of FIG. 9A.

FIG. 9C illustrates an inverse Fourier transform of a cusped featureproviding uniform scattering on a hemisphere.

FIGS. 10A and 10B show the effect of scattering by a regular array ofcusps and a random arrangement of cusps, respectively.

DETAILED DESCRIPTION

The conflicting requirements for optimal absorption of both infrared andvisible light energy are resolved as described in the followingdisclosure. Various techniques have been previously described for thetexturing of silicon solar cells. The present disclosure relates toirregularly textured silicon technology based solar cells andphotodetectors by nanometer- and micrometer-sized structures thatproduce a true diffuse scattering of desirable wavelengths of incidentradiation. While silicon is the preferred semiconductor base material,other elemental and compound semiconductors such as Ge, GaAs, CdTe,CuInGaSe, and others exhibiting a photoelectric effect may also benefitfrom the use of the present invention. Diffuse scattering or diffusereflection is an important concept and, as used herein (and compatiblewith a definition available from Wikipedia.org), is intended to mean thescattering or reflection of light from a surface such that an incidentray is reflected at many angles that can be described as arbitrary,rather than at just one precise angle, which is the case of specularreflection. If a surface is completely nonspecular, the reflected lightwill be evenly scattered over the hemisphere surrounding the surface.Materials can reflect diffusely if their surface is randomly rough onthe microscopical scale of the radiation being scattered or reflected.

Solar cells and photodetectors are typically comprised of diodes formedin a semiconducting material. The solar cell/photodiode disclosed hereinincludes a semiconductor bulk substrate with at least one photoelectricconversion diode layer disposed in the substrate and at least one lightanti-reflecting region formed in or optically near the semiconductorsubstrate surface on which the radiation is initially incident. Suchincident radiation is expected to occur at a range of angles relative tothe plane of the top surface of the semiconductor substrate, includingnormal to the top surface, and this radiation is considered to beavailable to the solar cell/photodiode. The light anti-reflecting regionincludes an air-substrate index of refraction matching layer and, insome embodiments, light diffusing layer with features configured toincrease the effective absorption efficiency and energy conversionefficiency of the solar cell/photodetector. The anti-reflecting featuresmay be cones, pyramids, pillars, protrusions and other like features,and, when such features are used for diffusion, are distributed in arandom fashion. It should be noted that any feature that produces thedesired diffusive light scattering is one that closely approximates aLambertian scattering surface at the desired wavelengths of radiation.Lambertian scattering is ideal diffuse scattering providing lightdistributed over the whole half sphere or solid angle of 2π sterradians.Manipulating the feature sizes, dimensions, etc. allows the lightanti-reflecting and light diffusing region to be tunable for a specificwavelength. Varying the material near or deposited upon theanti-reflecting and light diffusing region can also be used to enhancethese characteristics.

An embodiment of the solar cell disclosed herein includes asemiconductor bulk substrate with at least one photoelectric conversiondiode layer disposed in the substrate and at least one light diffusingregion formed in or optically near the semiconductor substrate surfaceon which the radiation is initially incident and a reflector disposedbehind the light diffusing region. The light diffusing features, asdiscussed above, are chosen from cones, pyramids, pillars, protrusionsand other like features, and combinations thereof. These features alsoclosely approximate a Lambertian light scattering surface.

The light diffusing regions are formed by methods known to those skilledin the art to produce micrometer or smaller features on or withinsubstrates of semiconductor material. These may be formed before, duringor after the formation of solar cells/photodetectors and are formed onsemiconductor substrates that are considered “thick”, e.g., having arange of thickness between 500 μm and 5 μm, such as may be the result ofthe sawing process of a semiconductor ingot. Or, are formed onsemiconductor substrates that are considered “thin”, e.g., having arange of thickness between 10 μm and 1 μm, such as may be the result ofa deposition process yielding microcrystalline silicon or amorphoussilicon or both. As employed in the present disclosure, the followingterms have the following meanings, compatible with those found atwikipedia.org. Crystalline, or single crystal, silicon is silicon inwhich the crystal lattice of the entire sample is continuous andunbroken with no grain boundaries. Multicrystalline silicon is a siliconsample in which segments of the silicon are composed of large crystalsthat may have different orientations with grain boundaries between thecrystals. When cut into substrates several different crystals may bevisible to the eye. Polycrystalline materials are made of a threedimensional mosaic of small, irregularly shaped crystals.Microcrystalline silicon is a form of silicon having a paracrystallinestructure; paracrystalline materials have only short and medium rangeordering in their lattice structures over micrometer dimensions.Amorphous silicon has no crystal structure and is disordered, or theatoms are arranged in no particular order.

Traditional solar cells, photodiodes, and photodetectors, designated inFIG. 1, use what is called FSI, or Front Side Illumination technology,where the radiant energy 106 is incident on a planar surface 101 of anabsorbing silicon substrate 102. In conventional CMOS imagers employinga photodetector, the substrates 102 are hundreds of micrometers thickbut epitaxial or silicon-on-insulator substrates are used to limit thedepth of absorption of the incident radiation and CMOS and CCD imagersare sometimes thinned to a few micrometers. Recently solar cells havebeen fabricated using thin films of microcrystalline silicon. As aresult of the thin active film thickness of these devices, theefficiency of the absorption of infrared and long wavelength visiblelight in thin silicon films has become important.

When the silicon is not strongly absorbing in the near infrared a backside textured surface works in conjunction with a totally internallyreflecting front side surface to best increase the absorption of nearinfrared photons. This results in multiple internal reflections andlight trapping.

An embodiment described herein for visible light, infrared, and nearinfrared, NIR, detectors and imagers and thin film solar cells frontside illumination and backside texturing. The backside texturing isparticularly designed to provide true diffusive scattering and, in onealternative embodiment has a transparent conductive oxide treated orpatterned to produce randomly positioned or otherwise arranged for ascattering property yielding a Lambertian scattering of energy of aselected band of wavelengths and a reflecting layer preferably formed ofa metallic material.

Consider now the effect of texturing on the near infrared wavelengthresponse; texturing will also change the absorption in the remainingpart of the visible light region but this will not be considered here. Atypical semiconductor substrate 102 is shown in cross section in FIG. 1.The back surface 103, i.e., the surface opposite the top surface 101, iscoated with a conductive oxide layer 104 that is regularly textured withgrooves or ridges or similar patterns to produce an internal reflectionthat is not specular. In the near infrared the index of refraction ofsilicon is η=3.42 and the reflectance is about R=30% from a singlesurface and transmittance through a single surface is T=70% for normalincident waves. The absorption coefficient of silicon is very low in thenear infrared. In FIG. 1 radiation under normal incidence, representedby arrow 106, is reflected from the first surface 101, and this is shownas arrow 107. There are successive reflections from both the back,represented by arrows 108 and 110, and internal reflections from thefront surface, arrow 111, resulting in a total transmittance, if thereis no reflective metal layer 105 disposed on the oxide layer 104, of

T _(tot)=(TT)(1+R ² +R ⁴+ . . . )=(TT)/(1−R ²)  (1)

This result has been obtained using the sum of a geometric series. Ifboth top and back surfaces are just polished silicon-air then thisresults in a total transmittance of 54% and a reflectance of 46%.

If the increase in the individual path lengths caused by the diffusescattering is neglected and, if the absorption coefficient is very low,then the total effective path length is determined by just the number ofreflections, and the total absorption can be shown to be

A=αd(1+R ₂)(1+R ₁ R ₂ +R ₁ ² R ₂ ²+ . . . )=αd(1+R ₂)/(1−R ₁ R ₂)  (2)

Here, α, is the absorption coefficient in reciprocal cm and, d, is thethickness of the sample in cm, and the effective increase in path lengthis Enh=(1+R₂)/(1−R₁R₂). The internal quantum efficiency, IQE, in theinfrared where the absorption in silicon is low is then, IQE=αd Enh. Theexternal quantum efficiency, EQE, is EQE=T₁ IQE and EQE=T₁ αd Enh.

If both sides of an infrared photo detector or thin film silicon solarcell are polished then T₁=T₂=0.70 and R₁=R₂=0.3 which gives Enh=1.4,IQE=1.4 αd and EQE=αd.

An embodiment that improves the infrared response has the top side 101polished but the back side 103 textured with an oxide 104 like siliconoxide or transparent conductive oxides 104 like zinc oxide, indiumoxide, or tin oxide, and a metal like aluminum or silver or mirrorreflector 105 behind. The texturing is realized in a fashion to producea true diffuse scattering, a Lambertian scattering, at the infraredwavelengths. This diffuse scattering layer/reflecting layer combination,in essence, yields an R₂=100%, a diffuse reflector. The reflectance ofthe polished front side to the scattered light radiation is determinedby solid angle considerations. Any incident light with an angle ofincidence greater than the critical angle θ, 109, will be totallyinternal reflected, 111. If the backside scattering is totally diffuseor Lambertian, the transmittance is then determined by the area of thesurface, πr², within the critical angle θ, 109, in this case 17° forsilicon and air. The radius of the circle is r=d sin(17), where, d, isthe thickness of the sample. This area is divided by the area of thehalf sphere, 2πd². If the backside scattering is totally diffuse thetransmittance of the front planar surface is then roughly T₁=3% and thereflectance R₁=97%. The path length enhancement factor can be very large

Enh=(1+R ₂)/(1−R ₁ R ₂)=66  (3)

This would result in an IQE=66 αd and an EQE=46, this is consistent withCotter's estimate defined as, 4η², where, η, is the optical index ofrefraction of the semiconductor, then for a backside textured siliconphotodetector 4η²=51 in the near infrared where the index of refractionfor silicon η=3.42. If the backside is a textured and truly diffusivescattering surface and a mirror like surface is used behind the backside, a very large enhancement of absorption in the near infrared can beachieved.

Aberle has described a related technique for thin solar cells. As an aidto understanding, FIG. 2 reproduces an isometric view of Aberle's thinsolar cell 200 with front side and backside electrical contacts 201 and202, respectively. Aberle used white paint as the backside reflector.Aberle's initial results show a large increase in the absorption by thesolar cell with the white paint as a back surface reflector but only amodest ten or twenty percent increase in the quantum efficiency, asshown herein in FIG. 3. These thin films are deposited on a glass plate,208, covered with silicon nitride, 207. A seed layer, 206, is used toform the microcrystalline absorbing layer, 205. Layer, 204, is a heavilydoped junction forming layer and 203 is a back surface reflector. Layer203 is thick only in Aberle's case, in FIG. 2 it is shown as a thinmetal reflector.

As shown in FIG. 3 the quantum efficiency, curve where a pigmentedbackside reflector, e.g. white paint, is used can be compared to a backsurface without the pigmented back side. They indicate that a twenty toforty percent enhancement in conversion efficiency can be obtained onthin film microcrystalline solar cells by adding more titanium dioxideto the white paint. However, thick layers of the order 80 micrometers ormore of paint are required on thin film solar cells 200 which inthemselves are only a few micrometers thick.

FIG. 4 shows the improvement realized with one embodiment of the presentinvention on thick silicon samples for a backside textured Lambertianscattering surface in the near infrared. The normal or conventionalresults for a single pass of a 100 μm thick epitaxial sample curve 401are compared to backside texturing with total internal reflections atthe front side and light trapping in curve 402. “Light trapping” meansthat the light is confined by internal reflections in the semiconductorlayer until it is absorbed. The enhancement factor observed at infraredwavelengths like 1100 nm is at least a factor of twenty. The 100 μmbackside textured sample, curve 402, appears like a 700 μm sample withno texturing that is measured also as curve 402. A comparison is beingmade between the thinner 100 μm sample and a thick 700 μm sample.Multiple internal reflections without texturing but with an ideal metalbackside reflector results in an enhancement factor from Eqn. 3 of aboutthree. The 700 μm thick sample with no texturing, curve 402, but abackside metal reflector will then have an apparent of effectivethickness of 2000 μm. A 100 μm sample could ideally have an enhancementof over 60 but in the practice this is limited by non-ideal losses sothan the actual enhancement is around twenty and both samples have anapparent thickness of 2000 μm. This results in a useful absorption insilicon out to 1100 μm or the indirect bandgap energy of silicon.

If the absorption in the silicon layer is not assumed to be small butrather taken into account it can be shown that the enhancement factorfor the internal quantum efficiency due to multiple reflections ismodified from Eqn. 3 and becomes

Enh=(1−exp(−αd))(1+R ₂exp(−αd))/(1−R ₁R₂exp(−2αd))  (4)

This allows a calculation of the responsivity, in terms of theelectrical current in Amperes per incident light power in Watts, ofsolar cells of different thickness, d, for different wavelengths, λ,since the absorption coefficient, α(λ), is a function of wavelength. Ifit is assumed that the backside is an ideal reflector, R₂=1.0, and theamount of diffusive scattering of the back surface varies from that of aplanar surface then the fraction of light reflected back from the frontsurface will vary. If the back surface is planar then there is onlyspecular reflection and, R₁=0.3, if the back surface is an idealLambertian diffusive surface then the fraction of light reflected backfrom the front surface will be very large, R₁=0.97. Several values of R₁are discussed herein for a diffuse reflector, these represent thefraction of light internally reflected back at the front surface. Forpurposes of the present invention, values of R₁≧0.9 are deemedparticularly useful. The enhancement in absorption described by Equation4 then varies with the fraction of light radiation reflected back fromthe front surface and thickness of the sample.

FIG. 5 compares the responsivity of a thick (d=200 μm) crystalline solarcell as the reflectivity, R₁, of the back surface is varied. When boththe top surface and the back surface are planar (R₁=0.3), the responseis indicated by curve 501, only 30% of the light is reflected back fromthe front surface and the estimated short circuit current for a solarcell of one square centimeter is 33 mA. The effect of varying R₁ isshown by the other curves, 502, 503, and 504, for which R₁=0.5, R₁=0.7and R₁=0.9, respectively. Thus, a Lambertian diffusive reflecting backsurface provides the optimum responsivity.

FIG. 6 is a graph of the calculated responsivity of one squarecentimeter thin film microcrystalline silicon solar cell with ascattering at the back surface and with a reflector. There can bevarying values of the amount of light reflected back from the frontsurface of 30%, 50% and 90%. If R₁=0.9, curve 601, then most of theenergy in the red and infrared incident radiation is absorbed and ashort circuit solar cell current of 25 mA is calculated. Thus, theconversion efficiency approaches that of thick crystalline solar cellsthat are one hundred times thicker. The other curves, 602 and 603, showthe effect of varying R₁ to values of 0.7 and 0.5, respectively.

A thin solar cell/photodetector is shown in FIG. 7. This solar celldiffers from the one described by Aberle in that a thin texturedLambertian scattering back surface 703 is backed by a thin reflectingsurface 709 rather than a thick layer of white paint a many timesthicker than the solar cell, up to 80 micrometers thick, as in aPigmented Diffuse Feflector (PDF) with embedded titanium oxidenanoparticles. The solar cell as disclosed herein is either acrystalline silicon p-i-n diode or a microcrystalline silicon p-i-ndiode. The semiconductor junction layer 701 at the back and thesemiconductor junction layer 702 at the front are any of conventionalimplanted or diffused junctions or, for heterojunctions, doped amorphoussilicon layers. For optimum infrared and long wavelength red response,the backside is textured with a Lambertian scattering layer 703, thescattering region may be due to a treatment of the silicon material orof the oxide layer, and backed by a reflecting layer 709, while thefront side is a reflective planar surface 704. As shown before a verylarge enhancement can be obtained in the infrared response by the totalinternal reflection of the incident infrared light, represented by arrow710, incident on the backside diffuse scattering surface 703 andreflector 709. Thin layers of microcrystalline silicon are used forsolar cells and multiple internal reflections used to increase theabsorption of infrared and the longer wavelengths of visible red light.Thus, an embodiment is an amorphous silicon, microcrystalline-amorphoussilicon thin film n-i-p heterojunction solar cell. The front side 704has an oxide or transparent conductive oxide layer 705 that isanti-reflecting to visible light, although this anti-reflecting propertymay be omitted in alternate embodiments, and electrical contacts 706.The backside has an oxide or transparent conductive oxide layer 707 andelectrical contacts 708.

An alternative embodiment is to use front side texturing for thesemiconducting substrate that provides diffusive scattering to infraredlight. This alternative results in increased path lengths and increasedabsorption for infrared radiation, particularly if the backside of thesubstrate is reflective or a reflective metal contact. The enhancementof the absorption of infrared light energy will not be as great as itcan be with backside diffusive scattering since the number of passes islimited; it is easier for the infrared light reflected from the back toescape out of the textured front than it would be to escape from aplanar front side. As a result the average number of passes of infraredlight that is not strongly absorbed is estimated to be limited to aboutthree; still, some significant increase in energy conversion efficiencyof solar radiation can be achieved since the individual path lengths ofscattered light are greater than the thickness of the substrate. Asbefore the textured front side that provides diffusive scattering toinfrared light can be covered with a layer of conductive or insulatingoxide that is anti-reflecting to visible light. This then also ensuresoptimum absorption of visible light.

FIG. 8 is a cross section of a stacked pair of solarcells/photodetectors 800, 700 which receive incident electromagneticradiation, illustrated by arrow 810, at a front surface of cell 800. Theband of electromagnetic radiation that encompasses visible light isprimarily absorbed, that is, converted to electricity, in cell 800. Theband of electromagnetic radiation that encompasses infrared and longwavelength red light is primarily absorbed in cell 700. The maximumbenefits of texturing can be achieved by using a structure consisting ofthe two stacked solar cells, as shown. As used herein the term “stacked”means two separate solar cells, one mounted atop the other with a gapbetween them. Consider the arrangement in which a semiconductor solarcell 700, as previously described in relation to FIG. 7, is orientedsuch that a second semiconductor solar cell 800 is interposed betweenthe cell 700 and the source of incident radiation. Herein, such anarrangement will deem solar cell 800 to be the top cell and solar cell700 to be the bottom cell. The bottom cell semiconductor junctions atthe back, 701, and front, 702, can either be conventional implanted ordiffused junctions or doped amorphous silicon layers to formheterojunctions. The top solar cell 800 has a textured front layer 801that transmits and allows absorption of visible light within top cell800, and a textured back layer 803. Layer 801 is textured to transmitvisible and infrared light and the texture of back layer 803 transmitsinfrared light. The bottom solar cell 700 has a smooth front layer 704and textured back layer 703 that scatters infrared light energy. Mostinfrared light, depicted as arrow 810, just passes through the top solarcell 800 and is not absorbed; the front textured layer 801 and backtextured layer 803 in the top solar cell 800 are designed to be anantireflective layers that just transmit infrared light. The bottomsolar cell 700 absorbs most of the infrared light energy due to multipleinternal reflections and trapping of the infrared light, as described inconjunction with FIG. 7. The planar front surface 704 is internallyreflective to infrared light that is scattered and reflected from thebackside. The planar front surface 704 has disposed thereon an oxide ortransparent conductive oxide 705 that has electrical contacts 706, andhas, at the option of the designer, anti-reflecting properties tovisible light. The backside of cell 700 has an oxide or transparentconductive oxide layer 707 and electrical contacts 708.

The front side of cell 800 has an oxide or transparent conductive oxidelayer 805 that may or may not be anti-reflecting to visible light,disposed on the textured front layer 801 and electrical contacts 806.The backside of cell 800 has an oxide or transparent conductive oxidelayer 807 disposed on textured back layer 803 and electrical contacts808. Each of the electrical contacts makes a very low resistanceconnection to the silicon and its associated oxide layer, that is,electrical contacts 706 electrically connect to oxide layer 705,electrical contacts 708 electrically connect to oxide layer 707,electrical contacts 806 electrically connect to oxide layer 805, andelectrical contacts 808 electrically connect to oxide layer 807.Electrical contact between the two cells 800, 700 is provided bybackside contacts 808 and front side contacts 706, respectively. Theefficiency of stacked or tandem cells 800 will be about 25% higher thana single cell.

A variation to the embodiment of FIG. 8 employs a stacked configurationof textured amorphous silicon, microcrystalline-amorphous silicon thinfilm n-i-p heterojunction solar cells. It is to be noted that thisconfiguration is different than the-tandem configuration described inthe aforementioned Choi patent. The present stacked configuration has alarge discontinuity in the index of refraction between the two cells800, 700 by virtue of having a space between the cells, which is alsouseful in enabling electrical connections between the cells; the Choitandem configuration is all one material and does not have this. Thisdiscontinuity in the index of refraction due to the space is responsiblefor multiple internal reflections in the bottom cell 700.

An alternative embodiment allows the cells to be one continuous piece ofmaterial, as in the Choi tandem cell configuration, without adiscontinuity in the index of refraction at the center but with a planarfront and textured back with reflector. This arrangement also results intotal internal reflections and light trapping and has not been disclosedby Choi.

Texturing of the backside layer of photodetectors or solar cells can beaccomplished by any one of several well known techniques, where theseinclude the removal of silicon material by chemical etchants, plasmaetching, porous silicon etchants, lasing to produce black silicon,anisotropic etches like KOH, texturization by sodium carbonate, maskingto produce nanostructures by isotropic etch of the silicon, and maskingtechniques including nanoimprint masks. Diffusive layers are most easilyproduced by etches, like porous silicon etches, that produce randomfeatures. Porous silicon structures have been described in Tsuo for thefront side of substrates to reduce reflectance of visible light.However, such structures have not been described for the backside ofsubstrates to produce diffusive reflectance of red and infrared light.As considered herein, diffuse reflective layers that are added to thebackside of photodetectors or solar cells are random features thatproduce diffusive layers at selected wavelengths for incidentelectromagnetic energy, where these structures include hemisphericalgrains of polysilicon, titanium oxide nanoparticles embedded in oxide,Atomic Layer Deposition (ALD) of metals, and hexagonal nanocystalsproduced by diffusion of metals like tungsten or tantalum into amorphoussilicon. Grains or nanoparticles are random features that producediffusive layers for incident light. If necessary to increasereflectivity the structures formed are covered with oxide or transparentconductive oxide and a reflective metal layer.

The backside texturing described herein preferably produces diffusivescattering of light, either forward diffusive scattering, reflecteddiffuse scattering or both. This diffuse scattering is to bedistinguished from specular reflection and reflections from regulargeometric structures and shapes. Regular geometric structures and shapeslike trenches, ridges, saw tooth patterns, or pyramids or etchstructures produced by KOH alone will only reflect or scatter light atspecific angles with respect to the incident light and is not diffusive.Although Mandelkorn describes a ground and silvered bottom glass coverplate separated from the substrate, Mandelkorn does not describestructures etched into or deposited on to the substrate. Groundstructures of regular sizes, such as those taught by Mandelkorn, are notdiffusive and because of separation from the substrate the lightreflected into the substrate will be refracted towards the normal andnot diffusive. The Green patent ridges and pyramids etched into thesubstrate, but these produce reflections only at specific angles and arenot diffusive. The Matsuyama patent discloses solar cells CVD depositedupon opaque substrates with linear recesses, unlike the present solarcells disclosed herein, which have random irregularies of various sizesetched or deposited on planar surfaces. The material at the backside ofthe cell or deposited on the backside of the solar cell disclosed hereinis reflective, not simply opaque. Diffuse scattering produced by randomfeatures occurs over a range of angles at the desired band ofwavelengths and in the present invention approaches the ideal Lambertianscattering. Ideal reflective Lambetrian scattering provides lightdistributed over the whole half sphere or a solid angle of 2πsterradians. Herein the use backside texturing provides diffusivereflective light scattering at the backside and planar front sidesurfaces to provide for total internal reflections at the front side andmultiple passes of red and infrared light in silicon solar cells. Thefront surface preferably has an anti-reflecting layer for visible light,but this layer does not significantly affect the reflectivity of thefront surface to infrared light.

Thus, a solar cell/photodetector has been described that providesoptimal conversion of visible light and infrared energy to electricity.Backside texturing with diffusive scattering and frontside internalreflection produce a large enhancement of conversion efficiency. Oneembodiment of the present invention uses a set of cusps etched into thebackside of solar cells to provide diffusive scattering.

With the teachings of Born and Wolf in mind, one realizes that slits canbe replaced by reflective strips in an absorbing material. Considerfirstly light of one particular wavelength, or wavenumber, k. For anarray of n strips of width, s=2a, and equal spacing, d, with the lightvertically incident, the intensity of the diffracted light on a screen,I, is proportional to:

I˜(H(ndkp/2)sinc(skp/2))²  (5)

sinc(x) is the ratio, sin(x)/x, and p=sin θ, θ is the angle from thevertical at which the light is diffracted. The formula for the intensityis valid if the observing screen is much farther away than, a or d, ornamely, the Fraunhofer diffraction regime. In this case, the diffractionpattern is equivalent to the Fourier transform of the diffractiongrating. For a single strip of finite width, the diffraction pattern hasthe well-known form of a sinc function. For multiple slits of finitewidth, the diffraction is a pattern of peaks and troughs of theaperture, n d, whose locations are described by the interferencefunction, H, and whose intensity is modulated by the sinc functionpattern arising from a single strip. The envelope of the intensities ofthe regular diffraction pattern observed on the screen is a sincfunction. (Alternative derivations to that in Born and Wolf are tofirstly recognize that a set of narrow strips are described by theconvolution of a series of delta functions with a single strip, so theFourier transform of the grating is the product of the transform of aseries of delta functions, which yields H times the Fourier transform ofa single strip the sinc function. This yields H˜sin(ndkp/2)/sin(dkp/2).Secondly another approach is to use the time shift theorem in Fouriertransforms, if the transform of a single element f(x) is, F(k), thetransform of each element displaced in time or in this case space by thespecific distance, d, will be f(x−d)=F(k) exp(−ikd). This lattertechnique works best in dealing with a series of elements at randomlocations.

Consider now a solar cell as shown in FIG. 9A, similar to FIG. 1, wherelight incident on the top or front surface 101′ passes through the celland strikes a textured 104′ reflective 105′ surface at the bottom orback surface 103′ of the solar cell. Lambertian scattering by a singleelement into a hemisphere will, as shown in FIG. 9B, result in aLorentzian intensity pattern 910, depicted in the intensity versusdistance graph of FIG. 9B for scattered light displayed on a flatscreen. Light will be scattered large distances in the “x” direction 911as a result of the substantially uniform intensity pattern on thehemisphere 903. In order to achieve Lambertian scattering, one must findthe inverse Fourier transform of the Lorentzian and use this for theshape of a single element providing scattering or for each elementproviding diffraction in an array. The Fourier transform of each elementproviding scattering will then give uniform illumination of a hemisphere903 and a Lorentzian shape 910 on the flat screen. The inverse Fouriertransform of a Lorentzian is a “cusp” 930 as shown in FIG. 9C and in thecusped features (in exaggerated form for clarity) in FIG. 9A. A seriesor array of such regular cusped features, disposed with the base portionof the cusped feature toward the reflective layer 105′ of bottom surface103′ (away from the front surface 101′), results in a series of dots oflight being presented at the top surface 101′ with an intensitydescribed by a Lorentzian envelope over a wide range of distances. Eachdot will have the shape and location described by the interferencefunction, H, of the aperture defining the region occupied by the“cusps.” This regular array of cusped features 1001 disposed on asurface, like surface 103′, produces a regular diffraction pattern ofdots 1002 on the front surface 101′ as shown in FIG. 10A (c.f. Born andWolf). Likewise a random array 1010 of cusped features formed on asurface, like surface 103′, will also work as well; these produce arandom array of dots 1020 on the front surface 101′ or a “fuzzy” patternas shown in FIG. 10B (c.f. Born and Wolf).

In the case of a solar cell with very wide features there is essentiallyno aperture. A regular array or random arrangement of cusped features onthe backside results in Lambertian scattering from each elementseparately and optimal scattering. In the case of solar cells there needbe no concern about the accumulative effect of a large number of “cusps”and whether or not they form a regular diffraction pattern. We need onlybe concerned here that the scattering from each element be Lambertian,this will insure a maximum of total internal reflection back from thesilicon side of the front surface. The scattering from each cusp will bea diffusive or nearly diffusive scattering. The embodiments of thepresent invention illustrated in FIGS. 7 and 8, in alternativeembodiments, employ the cusped-featured surface as the scattering layer703. Moreover, in a stacked arrangement, as depicted in FIG. 8, thefront textured layer 801 and the back textured layer 803 are of cuspedfeature design, in which the cusp dimensions are tuned to transmitinfrared light.

A random array of such “cusps”, in one embodiment, are etched into thebackside of a silicon wafer by porous silicon and metal catalyst etchessuch as described by S. W. Chang et al. or S. Brieger et al. In oneembodiment of the present invention, the porous silicon or metalcatalyst etched vertical holes are etched with a conventional isotropicsilicon etch to round off the shape corners resulting in a cusp likestructure. A thin layer of oxide is either grown or deposited or anotherdielectric deposited and the backside covered by a reflective metal. Inthis manner a random array of cusp like scattering centers can be formedon the back of silicon solar cells. Alternative regular arrays areformed by using the self-assembly features of metal catalyst etchants.

Of course, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent disclosure. Numerous modifications and alternative arrangementsmay be devised by those skilled in the art without departing from thespirit and scope of the present disclosure is intended to cover suchmodifications and arrangements. In particular the term “cusps” describesnot only those following the exponential functional form but also anystructure exhibiting a similar “cusp-like” shape. Thus, while thepresent disclosure has been described above with particularity anddetail in connection with what is presently deemed to be the mostpractical embodiments of the disclosure, it will be apparent to those ofordinary skill in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

1. A semiconductor solar cell or photodetector having improved incidentradiation absorption, comprising: a first silicon substrate having asubstantially planar first surface available to the incident radiationand a second surface; a layer disposed at said first surface, said layerbeing transparent to visible light and infrared wavelengths of incidentradiation and internally reflective to infrared wavelengths of radiationscattered within said first silicon substrate; and a textured layerdisposed at said second surface, said textured layer comprising aplurality of cusped features disposed with bases of said plurality ofcusped features arranged away from said first surface and adapted toform a diffusive radiation scattering layer for infrared wavelengths ofradiation.
 2. The semiconductor solar cell or photodetector of claim 1further comprising a reflecting layer disposed at said textured layerand spaced apart from said second surface by said textured layer,whereby infrared wavelengths of radiation are returned through saidtextured layer toward said first surface.
 3. The semiconductor solarcell or photodetector of claim 1 wherein said silicon substrate furthercomprises single crystalline silicon or multi-crystalline silicon of arange of thickness between 500 micrometers to 5 micrometers.
 4. Thesemiconductor solar cell or photodetector of claim 1 wherein saidsilicon substrate further comprises deposited microcrystalline siliconof a range of thickness between 10 micrometers to 1 micrometer.
 5. Thesemiconductor solar cell or photodetector of claim 1 wherein saidsilicon substrate further comprises deposited amorphous silicon of arange of thickness between 10 micrometers to 1 micrometer.
 6. Thesemiconductor solar cell or photodetector of claim 1 wherein saidsilicon substrate further comprises both deposited microcrystallinesilicon and deposited amorphous silicon of a range of thickness between10 micrometers to 1 micrometer, arranged as one of either (a) anamorphous layer and then a microcrystalline layer and an amorphouslayer, or (b) an amorphous layer, and a microcrystalline layer.
 7. Thesemiconductor solar cell or photodetector of claim 2 wherein saidreflecting layer comprises an oxide substantially transparent toinfrared wavelengths of radiation and further comprises a reflectivemetallic layer.
 8. The semiconductor solar cell or photodetector ofclaim 1 wherein said textured layer further comprises one of a regulararray of cusped features and a random array of cusped features.
 9. Thesemiconductor solar cell or photodetector of claim 8 wherein said one ofa regular array and a random array further comprises an etched one of aregular array and a random array.
 10. The semiconductor solar cell orphotodetector of claim 1 further comprising first and secondmicrocrystalline silicon junction diode materials disposed substantiallyin planes parallel to said first surface.
 11. The semiconductor solarcell or photodetector of claim 1 wherein said layer disposed at saidfirst surface further comprising a layer anti-reflective to visiblelight wavelengths of radiation.
 12. The semiconductor solar cell orphotodetector of claim 1 further comprising: a second silicon substratehaving a substantially planar second substrate first surface availableto the incident radiation and a second substrate second surface, saidsecond substrate second surface disposed adjacent said firstsemiconductor substrate first surface; a second substrate layer disposedat said second substrate first surface, said second substrate layercomprising a layer anti-reflective to infrared and visible lightwavelengths of incident radiation; a second substrate second texturedlayer disposed at said second substrate second surface, said secondsubstrate textured layer comprising a layer anti-reflective for infraredwavelengths of radiation.
 13. The semiconductor solar cell orphotodetector of claim 12 further comprising a stacked second and firstsilicon substrates wherein said second substrate second surface and saidfirst semiconductor substrate first surface are separated by a gap thatcreates a discontinuity of index of refraction at least greater than1.0.
 14. The semiconductor solar cell or photodetector of claim 13wherein said second silicon substrate comprises second substrate firstand second amorphous silicon junction diode materials disposedsubstantially in planes parallel to said second substrate first surface.15. The semiconductor solar cell or photodetector of claim 14 furthercomprising a stacked second and first substrates wherein said secondsubstrate first and second surface, and first substrate first surfaceare planar and first substrate first surface and second substrate secondsurface are continuous semiconducting material and the first substratesecond surface is textured.
 16. A stacked semiconductor solar cell orphotodetector having improved incident radiation absorption, comprising:a second silicon substrate, said second substrate having a first surfaceavailable to the incident radiation and a second surface upon which isdisposed a conductive infrared transparent layer upon which a firstelectrical contact is disposed, said second substrate beingsubstantially transparent to infrared wavelengths of the incidentradiation; a first silicon substrate having a substantially planar firstsubstrate first surface disposed opposite said second substrate secondsurface across a gap and spaced apart from said second substrate secondsurface by said gap such that a difference between the index ofrefraction of said first substrate and said gap results in infraredwavelength energy internally scattered toward a first substrate firstlayer being reflected back into said first substrate; and said firstsubstrate comprising a first substrate conductive infrared transparentlayer disposed on said first substrate first surface, upon which asecond electrical contact is disposed and arranged to make electricalconnection with said first electrical contact, a first substratetextured layer comprising a plurality of cusped features disposed atsaid first substrate second surface with bases of said plurality ofcusped features arranged away from said first substrate first surfaceand adapted to form a diffusive radiation scattering layer for infraredwavelengths of radiation
 17. The stacked semiconductor solar cell orphotodetector of claim 16 further comprising a first substratereflective layer disposed on said first substrate textured layer,whereby infrared wavelengths of radiation are returned through saidfirst substrate textured layer toward said first semiconductorsubstrate.
 18. The stacked semiconductor solar cell or photodetector ofclaim 16, wherein said second substrate further comprises: a texturedfirst layer disposed at said first surface, said layer being transparentto visible light and infrared wavelengths of incident radiation; atransparent second layer disposed at said textured first layer; atextured third layer disposed at said second surface, said texturedlayer being transparent to infrared wavelengths of incident radiation;and a transparent fourth layer disposed at said textured third layer.19. The stacked semiconductor solar cell or photodetector of claim 16,wherein said first substrate further comprises both depositedmicrocrystalline silicon and deposited amorphous silicon of a range ofthickness between 10 micrometers to 1 micrometer, arranged as one of (a)an amorphous layer and then a microcrystalline layer and (b) anamorphous layer, a microcrystalline layer, and another amorphous layer.20. The stacked semiconductor solar cell or photodetector of claim 18,wherein said first substrate reflecting layer comprises an oxidesubstantially transparent to infrared wavelengths of radiation and saidreflective metallic layer.
 21. The stacked semiconductor solar cell orphotodetector of claim 15, wherein said first substrate textured layerplurality of cusped features further comprises one of a regular array ofcusped features and a random array of cusped features.
 22. A method ofmanufacture of a semiconductor solar cell or photodetector with improvedincident radiation absorption, comprising the steps of: disposing afirst layer, transparent to visible light and infrared wavelengths ofincident radiation and internally reflective to infrared wavelengths ofradiation, on a substantially planar first surface of a first siliconsubstrate; disposing a second layer on a second surface, substantiallyparallel to said first surface, of a first silicon substrate; andtexturing said second layer into a plurality of cusped features disposedwith bases of said plurality of cusped features arranged away from saidfirst surface, whereby a diffusive radiation scattering layer forinfrared wavelengths of radiation is formed.
 23. The method of claim 22wherein said texturing step further comprises the step of etching saidsecond layer into said plurality of cusped features.
 24. The method ofclaim 22 wherein said texturing further comprises the step of arrangingsaid plurality of cusped features into one of a regular array of cuspedfeatures and a random array of cusped features.
 25. The method of claim22 further comprising the step of disposing a reflecting layer at saidtextured layer and spaced apart from said second surface by saidtextured layer, whereby infrared wavelengths of radiation are returnedthrough said textured layer toward said first layer.